Secondary batteries are often provided to users in the form of rechargeable battery packs which can be repeatedly used. In particular, in order to protect users and electronic appliances, lithium ion secondary batteries having a high volumetric energy density typically include several protective circuits incorporated in battery packs for over-charging protection and over-discharging protection to interrupt the output of the battery pack under predetermined conditions.
Some of these protective elements use an FET switch incorporated in a battery pack to turn ON/OFF the output, for over-charging protection or over-discharging protection of the battery pack. However, even in the cases of the FET switch being short-circuited and damaged for some reason, a large current momentarily flows caused by a surge such as a lightning surge, and an abnormally decreased output voltage or an excessively high voltage occurs in an aged battery cell, the battery pack or the electronic appliance should prevent accidents including fire, among others. For this reason, a protective element is used having a fuse which interrupts a current path in accordance with an external signal so as to safely interrupt the output of the battery cell under these possible abnormalities.
As shown in FIGS. 10 (A) and (B), there has been proposed a protective element 80 of a protective circuit for a lithium ion secondary battery in which a meltable conductor 83 is connected as a part of a current path between first and second electrodes 81, 82, and this meltable conductor 83 in the current path is blown by self-heating due to an overcurrent or by a heat-generating element 84 provided in the protective element 80.
In particular the protective element 80 includes an insulating substrate 85, a heat-generating element 84 laminated on the insulating substrate 85 and covered with an insulating member 86, a first and a second electrodes 81, 82 formed on the both ends of the insulating substrate 85, a heat-generating element extracting electrode 88 laminated on the insulating member 86 and overlapping the heat-generating element 84, and a meltable conductor 83 the both ends of which are connected to the first and second electrodes 81, 82, respectively, and the central portion of which is connected to the heat-generating element extracting electrode 88.
In the protective element 80, when an abnormality such as over-charging or over-discharging is detected, current flows through the heat-generating element 84 and the heat-generating element generates heat. The meltable conductor 83 is melted by this heat and gathers on the heat-generating element extracting electrode 88 to interrupt the current path between the first and second electrodes 81, 82.